1. Field of the Invention
The present invention relates to a solid-state image sensing device and a method of driving the same image sensing device, and more specifically to a solid-state image sensing device having field memories adjoining a photosensitive region and being capable of reading two-line signal charges independently.
2. Description of the Prior Art
Among the solid-state image sensing devices of charge transfer type, a CCD area sensor, for instance, can obtain high resolution video signals, so that this sensor is widely used in broadcasting CCDs, high picture quality CCDs, electronic still camera CCDs, etc.
In the conventional solid-state image sensing device used as an area sensor which can read two-line image signals simultaneously, a first accumulation region and a second accumulation region are formed on both sides of a photosensitive region as field memories. Further, in at least one of the accumulation regions, a cyclic transfer path where a plurality of transfer stages are connected into a loop circuit is provided so that the arrangement sequence of photosensitive elements (pixels) can be changed. The signal charges accumulated in these accumulation regions are transferred along a first horizontal transfer path and a second horizontal transfer path, respectively and further detected by a first charge detecting circuit and a second charge detecting circuit both provided at the transfer ends, respectively. Thus, in general, the signal information (charges) of all the vertical pixels can be obtained within a single field period.
In the above-mentioned structure of the conventional solid-state image sensing device, after a first integration time setting pulse has been applied, in response to a first field shift (FS) pulse, signal charges are read from photosensitive pixels of odd ordinal numbers (2n-1) (n=1, 2, . . . ) counted from above in the vertical direction of the photosensitive region, and the read signal charges are transferred to the first accumulation region in response to a first field transfer pulse. Further, after a second integration time setting pulse has been applied, in response to a second field shift (FS) pulse, signal charges are read from photosensitive pixels of even ordinal numbers 2n (n=1, 2, . . . ) counted from above in the vertical direction of the photosensitive region, and the read signal charges are transferred to the second accumulation region in response to a second field transfer pulse. In the case of the above-mentioned transfer, there exists a time difference between the first and second FS pulses, which is equal to a time difference between the first and second integration time setting pulses. The influence of this time difference cannot be disregarded when the signal charge accumulation time (integration time) becomes shorter than a predetermined value.
Further, in the case of the solid-state image sensing device of such a type that all the charges are swept off to the semiconductor substrate prior to the integration start, after an integration time setting pulse has been first applied to the semiconductor substrate, in response to a first field shift (FS) pulse, signal charges are read from photosensitive pixels of odd ordinal numbers (2n-1) (n=1, 2, . . . ) counted from above in the vertical direction of the photosensitive region, and the read signal charges are transferred to the first accumulation region in response to a first field transfer pulse. Thereafter, after a second integration time setting pulse has been applied, in response to a second field shift (FS) pulse, signal charges are read from photosensitive pixels of even ordinal numbers 2n (n=1, 2, . . . ) counted from above in the vertical direction of the photosensitive region, and the read signal charges are transferred to the second accumulation region in response to a second field transfer pulse.
In the case of the above-mentioned transfer, there exists a time difference between the integration time (from the integration time setting pulse to the first FS pulse) and the signal charge accumulation time (from the integration time setting pulse to the second FS pulse), which is equal to a time difference between the first and second FS pulses. Therefore, the influence of the time offset cannot be disregarded when the signal charge accumulation time (integration time) becomes shorter than a predetermined value.
As described above, in the conventional solid-state image sensing devices, since signal charges are read by two charge detecting circuits for each field, where the integration time shorter than a predetermined time period is set, a big difference in image signal intensity is developed due to a difference in time between the lines or due to a difference in the signal charge accumulation time, with the result that there exists a problem in that the motion picture quality is deteriorated. For instance, here the assumption is made that the time difference between the above-mentioned first and second integration time setting pulses is equal to a 5-line horizontal transfer time (5H). In this case, if the integration time is 1/60 seconds, the picture quality is almost not degraded. However, the integration time is reduced down to about 1/5000 seconds which is, for example, obtained when a high speed electronic shutter is used, since the intensity of the obtained signal charge is low, the picture quality is markedly deteriorated. Further, since the S/N ratio decreases with decreasing accumulation quantity (integration value) of the signal charges of the first and second pixel groups, the picture quality is deteriorated accordingly.